During the manufacturing of web or sheet material, differential tensions can form across the width of the web or sheet material, for example, from thermal or length variations. These differential tensions cause differential strains or distortions to exist in the web or sheet material, resulting in the web or sheet material exhibiting distortions or non-planarity when positioned on a flat surface. The web or sheet material can exhibit non-flatness characteristics such as bumps, wariness at the edges, or ripples across the width. FIG. 1 shows an exaggerated view of an unflat sheet 10 exhibiting wariness at the edges and FIG. 2 shows an exaggerated view of unflat sheet 10 exhibiting bumps in a central portion of the sheet.
As mentioned above, these differential tensions can result from length variations, i.e., internal material length difference in the web or sheet material. For example, referring to FIG. 3, if sheet 10 were cut into a plurality of strips 12, any strip 12a which exhibits a non-flatness characteristic (e.g., bump, wariness, or ripple) would have a length greater than (i.e., would be longer than) a strip 12b which does not exhibit a non-flatness characteristic.
Such internal length differences may be of particular concern when the sheet is used in a particular application. For example, in the printing industry or graphics art field, a final image may be generated using several different overlays. That is, several sheets each having a portion of the final image may be separately disposed in overlapping/overlaying registration onto each other to form the final image. If one of the sheets has sufficient internal length differences, the portions (of the final image) from the separate sheets will not properly register to each other. Consequently, the a final image that is produced may be unacceptable or unusable for its intended purpose.
Several methods have been employed to determine if a sheet includes non-flatness characteristics. For example, a user may conduct a simple visual inspection of the sheet. Such a visual inspection can be conducted by laying the sheet (or the section of the web material) on a flat surface or table, and then manually counting the number of bumps or waves in a given length. Such a visual inspection is time-consuming and prone to errors. In addition, this visual inspection does not provide a method for measuring the internal length differences in a web or sheet material.
An on-line web planarity measurement apparatus may provide an indication of whether the sheet includes non-flat characteristics. Such an apparatus is disclosed in U.S. Pat. No. 5,678,447 (Graff), commonly assigned and incorporated herein by reference. As noted above, the planarity of the web or sheet material is related to internal length differences. However, this reference does not provide a method for measuring the internal length differences in a web or sheet material.
In another method, the sheet is cut into individual strips, and then each strip length is measured. Such a method is accomplished by laying a section of the web material on a flat surface or table and then cutting a sheet from the web material. The resulting sheet has a uniform length across its width such that, as shown in FIG. 3, if the sheet is measured in it's full width form, the length X of the sheet is the same at every position Y across the sheet. The resulting sheet is then cut/slit into plurality of strips 12. Each strip is then disposed within a mechanical apparatus to measure it's length. Such a method is time-consuming and prone to errors. In addition, many mechanical apparatus do not have the resolution required to measure the minute differential lengths necessary for Applicant's application, which are in the range of microinches. For example, if a raised area of the sheet has a dimension 0.005 inches from the flat surface with the length of the raised area being 3 inches in length in the direction of the measurement (i.e., in the X direction; the direction of the cut/slit), the additional length of material in the sheet is about 0.000023 inches (23 microinches).
Accordingly, while each of these methods may have achieved certain degrees of success in their particular applications, a need continues to exist for an apparatus and method for measuring the relative differential length of a web or sheet material. The apparatus should be suitable for measuring minute differential lengths, and the method should not be time-consuming or prone to errors. In addition, the method should not adversely affect the web or sheet material whose differential length is being measured.